To buy life insurance on your grandmother you need grandma's permission, and cooperation.
Also, if she's not in good health, you're going to be very limited about how much life insurance a company will be willing to sell.
In addition, she may need to answer some health questions, and maybe take a physical exam.
There are life insurance companies that may offer life insurance for your grandmother without any exam, such as, Globe life, Mutual of Omaha, or Colonial Penn.
Globe Life Insurance offers people age 78 and under up to $30,000 of simplified life insurance with no medical exam required. There are some health questions.